1. Field of the Invention
The present invention relates to an accumulator used as a pressure accumulating apparatus or a pulse pressure damping apparatus or the like. The accumulator in accordance with the present invention is used, for example, in a hydraulic piping or the like in a vehicle such as a motor car or the like.
2. Description of the Conventional Art
Conventionally, there has been known an accumulator structured such that a bellows is arranged in an internal space of an accumulator housing provided with an oil port connected to a pressure piping so as to compart the internal space into a gas chamber sealing high pressure gas therein and a liquid chamber communicating with a port hole of the oil port. The accumulator includes a type that an inner peripheral side of a bellows 51 is set to a gas chamber 55 and an outer peripheral side thereof is set to a liquid chamber 56 by fixing the other end (a fixed end) 51b of the bellows 51, in which a bellows cap 52 is attached to one end (a floating end) 51a, to an end cover 54 in an upper portion of a housing 53, as shown in FIG. 18 (which is called as “inner gas type” since the gas chamber 55 is set in the inner peripheral side of the bellows 51, refer to patent document 1), and a type that the outer peripheral side of the bellows 51 is set to the gas chamber 55 and the inner peripheral side thereof is set to the liquid chamber 56 by fixing the other end (the fixed end) 51b of the bellows 51, in which the bellows cap 52 is attached to one end (the floating end) 51a, to an oil port 57 in a lower portion of the housing 53, as shown in FIG. 19 (which is called as “outer gas type” since the gas chamber 55 is set in the outer peripheral side of the bellows 51, refer to patent documents 2 or 3).
In this case, in the accumulator connected to the pressure piping of an equipment, if an operation of the equipment stops, liquid (oil) is discharged little by little from a port hole 58, and in the inner gas type accumulator in FIG. 18, the bellows 51 elongates little by little on the basis of sealed gas pressure in accordance with this, whereby the bellows cap 52 comes down little by little so as to come into contact with a seal 59, and comes to a so-called zero-down state. Further, in the outer gas type accumulator in FIG. 19, the bellows 51 contracts little by little on the basis of the sealed gas pressure in accordance with this, whereby the bellows cap 52 comes down little by little, and the seal 59 provided on a lower surface of the bellows cap 52 comes into contact with an opponent member 60 so as to come to a so-called zero-down state. Further, in this zero-down state, since a part of liquid is sealed in the liquid chamber 56 (a space between the bellows 51 and the seal 59) by a seal 59, and pressure of the sealed liquid is balanced with gas pressure in the gas chamber 55, it is possible to inhibit an excessive stress from being applied to the bellows 51 so as to cause an abnormal deformation.
However, in the case that the zero-down is caused by the operation stop at low temperature, and the temperature rises in this state, the liquid sealed in the liquid chamber 56 and the charged gas thermally expand respectively, and their pressures rise respectively. In this case, a rising rate of the pressure is higher in the liquid in comparison with the charged gas, however, since a pressure receiving area in the bellows cap 52 is set smaller in comparison with the charged gas, the bellows cap 52 does not move until the liquid pressure becomes considerably higher than the gas pressure. Accordingly, there is a case that a great pressure difference coming to about some MPa is generated between the liquid pressure and the gas pressure in the inner and outer sides of the bellows 51, and if the great pressure difference is generated as mentioned above, there is a risk that the bellows 51 is abnormally deformed, or the seal 59 is damaged.
Reference is made to Japanese Unexamined Patent Publication No. 2005-315429, Japanese Unexamined Patent Publication No. 2001-336502, and Japanese Unexamined Patent Publication No. 2007-187229.
Further, since an accumulator shown in FIG. 20 is the outer gas type accumulator similarly to the accumulator in FIG. 19, and further has such a peculiar structure that an auxiliary liquid chamber 71 is provided in an inner peripheral side of the bellows 51, and a piston 72 with a piston seal 73 is inserted into the auxiliary liquid chamber 71 so as to allow free stroke, the following disadvantages are pointed out (refer to patent document 4).
(i) The bellows 51 can be expanded only at a volumetric capacity of the auxiliary liquid chamber 71 (if the volumetric capacity of the auxiliary liquid chamber 71 is increased, the contraction of the bellows 51 is limited, and if the chamber 71 is made small, an amount of liquid for expanding the bellows 51 becomes small, and an amount of expansion can not be increased).
(ii) Since the stroke is done in a state in which the piston 72 is sealed by the piston seal 73, a slip resistance caused by a seal surface pressure is high, and a motion of the bellows 51 slows down at the loss amount (a function serving as the accumulator is lowered).
Reference is made to Japanese Unexamined Patent Publication No. 2003-278702.
Further, the following patent document 5 discloses an accumulator structured such that a secondary piston is coupled to a bellows cap via a secondary bellows, however, the following disadvantage is pointed out in this conventional art.
(iii) Since the contraction of the bellows is done in a state in which the secondary bellows expands at a time of the zero-down, and the contraction of the bellows stops at the stage that the secondary piston reaches the lowest surface, it is impossible to secure a sufficient expanding stroke of the bellows.
Reference is made to Japanese National Publication of Translated Version No. 2005-500487.